Instrument: Spectrograph

The MEGARA spectrograph will be located on GTC Nasmyth A platform
while the optical fiber bundles will be routed from the Folded
Cassegrain F focus to the entrance of the spectrograph at the
pseudo-slit position. MEGARA has a fully refractive optical system.
The spectrograph is composed by a pseudo-slit, where the fibers are
placed simulating a long slit 119 mm length and with a RoC of 1075
mm. Then the light passes through the collimator, which is composed
by 5 lenses and has a focal length of 483 mm, and a slit shutter
placed behind the first collimator lens. The geometry is fixed for
all spectral resolution modes and it is 68 degrees between
collimator and camera. The pupil has 160 mm diameter and it is the
location for the VPH-gratings (see figure below).

The spectrograph subsystems include the pseudo-slit, with a
XY mechanism that allows (1) exchanging among the
three positions (LCB, MOS, OPEN) and (2) moving towards
and away the first lens of the collimator for focusing;
the shutter; the cryostat, the pupil
elements at their mechanism, and the camera,
which is joined to the cryostat through the last lens, through the
last lens, with the CCD detector inside. The cryostat is based on
liquid nitrogen. The detector is a deep-depleted Silicon 4k x 4k E2V
CCD231-84 15μm pixels and 4 outputs with astro-2 coating. The CCD
is < 3e- read noise and has excellent QE across almost the whole
visible spectrum. In order to minimize electronic cross-talk between
amplifiers we will read using two diagonally-opposed amplifiers.

The MEGARA spectrograph provides three ranges of spectral
resolution: low (LR), medium (MR) and high (HR), with excellent
transmission thanks to the use of VPHs. The MEGARA VPHs
characteristics are summarized in the Table below.

VPH Name

Set-up

R (FWHM)

λ1 - λ2
(Å)

λc (Å)

Δλ (@ λc)
(Å)

Δv (km/s)

lin res Å/pix

1

VPH405-LR

LR-U

6028

3653 - 4386

4051

0.672

50

0.17

2

VPH480-LR

LR-B

6059

4332 – 5196

4800

0.792

49

0.20

3

VPH570-LR

LR-V

6080

5143 – 6164

5695

0.937

49

0.23

4

VPH675-LR

LR-R

6099

6094 – 7300

6747

1.106

49

0.28

5

VPH799-LR

LR-I

6110

7220 – 8646

7991

1.308

49

0.33

6

VPH890-LR

LR-Z

6117

8043 - 9630

8900

1.455

49

0.36

7

VPH410-MR

MR-U

12602

3917 - 4277

4104

0.326

24

0.08

8

VPH443-MR

MR-UB

12370

4225 – 4621

4431

0.358

24

0.09

9

VPH481-MR

MR-B

12178

4586 – 5024

4814

0.395

25

0.10

10

VPH521-MR

MR-G

12035

4963 – 5443

5213

0.433

25

0.11

11

VPH567-MR

MR-V

11916

5393 - 5919

5667

0.476

25

0.11

12

VPH617-MR

MR-VR

11825

5869 - 6447

6170

0.522

25

0.13

13

VPH656-MR

MR-R

11768

6241 - 6859

6563

0.558

25

0.14

14

VPH712-MR

MR-RI

11707

6764 - 7437

7115

0.608

26

0.15

15

VPH777-MR

MR-I

11654

7382 - 8120

7767

0.666

26

0.17

16

VPH926-MR

MR-Z

11638

8800 - 9686

9262

0.796

26

0.20

17

VPH665-HR

HR-R

18700

6445 - 6837

6646

0.355

16

0.09

18

VPH863-HR

HR-I

18701

8372 - 8882

8634

0.462

16

0.12

The wavelength coverage and spectral resolutions achieved by each
VPH in the LCB and MOS modes is summarized in the figure below.

In order to accommodate and exchange the gratings, the MEGARA
spectrograph has a Pupil Mechanism composed by the VPH wheel and a
insertion mechanism. The wheel can host 11 VPHs mounted
simultaneously. The mechanical mount of each VPH is screwed to a
platform.